(1) Field of the Invention
The present invention relates to a bottom abutment device for a rotorcraft rotor, to a rotorcraft rotor having the bottom abutment device, and to a rotorcraft provided with such a rotor.
The invention is thus situated in the technical field of rotorcraft rotors. More specifically, the invention is situated in the field of abutments fitted to such rotors in order to limit the flapping movement of lift assemblies carried by the rotor.
(2) Description of Related Art
Specifically, a rotorcraft usually has at least one lift rotor. The lift rotor contributes to providing the rotorcraft at least with lift and possibly also with propulsion.
A lift rotor may comprise a hub that is rotated by a mast. The hub then carries at least two lift assemblies. Consequently, each lift assembly has a blade that is connected to the hub by at least one retention and mobility member. In particular, each blade may comprise a lift element fastened to a cuff, or indeed it may comprise a lift element having an incorporated cuff.
For example, a retention and mobility member may include a hinge referred to as a “spherical abutment”. Each spherical abutment may have one strength member fastened to the hub and another strength member fastened to a cuff, the cuff being optionally incorporated in a lift element.
The retention and mobility members are fitted to respective ones of the blades at their roots in order to allow them to be moved by an operator. By way of example, mounting blades so that they are movable relative to the hub enables a pilot of the rotorcraft in flight to give rise to collective or cyclic variations in the pitch of the blades so as to influence the behavior of the rotorcraft with respect to its lift and/or propulsion.
The ability of the blades to move relative to the hub allows them to move with lead/lag movements, with pitch movements, and with upward and downward flapping movements. The notions of “up” and “down” should be considered in the direction in which the axis of rotation of the rotor extends. When the rotor is rotating at nominal speed, the blades are spontaneously taken upwards under the effects of centrifugal force and of the lift that they generate. When the rotor is stopped, the blades are subjected only to the force of gravity and they are therefore naturally taken downwards under the effect of their weight.
A rotor is thus commonly fitted with an abutment mechanism for limiting the flapping movement of the blades, in particular under the effect of forces external to the rotorcraft.
In one embodiment, the abutment mechanism may comprise, for each lift assembly, both a bottom abutment device and a top abutment device. A bottom abutment device is also known as a “bottom abutment” or “butée basse” in French language and a top abutment device is also known as a “top abutment” or “butée haute” in French language.
The bottom abutment device and the top abutment device respectively include a bottom abutment member and a top abutment member. The bottom abutment member and the top abutment member form obstacles to the individual flapping paths of the blades, respectively downwards and upwards. Furthermore, the bottom abutment device and the top abutment device include respective bottom and top abutment tracks carried by each lift assembly. For example, a strength member of a spherical abutment carries an abutment track in the form of a stop skid.
In the event of a blade performing excessive flapping movement, an abutment track comes into contact with an abutment member, thereby limiting the flapping movement of the blade.
The abutment mechanism serves in particular to limit the flapping movement of each lift assembly while starting the rotor, and more particularly when in the presence of wind. The abutment mechanism may also limit the flapping of the blades during an operation of folding the blades. In particular in the presence of spherical abutments hinging a blade to a hub, the abutment mechanism tends to preserve the spherical abutment when on the ground by avoiding a flapping movement of a blade in the absence of centrifugal force giving rise to the component elements making up the spherical abutment becoming mutually separated.
A known bottom abutment device comprises a bottom abutment member in the form of a bottom abutment ring. The bottom abutment ring is common to all of the lift assemblies. The bottom abutment ring is arranged around the axis of rotation of the rotor. Such a bottom abutment ring is then referred to as a “reciprocating” ring or “anneau réciproque” in French language.
The low abutment ring is movable only radially in a U-shaped groove of a retention ring.
On the ground, while rotating at low speed, or when stopped, the lift assemblies come simultaneously into contact with the low abutment ring, thereby mutually preventing one another from moving.
Document FR 2 671 049 describes a bottom abutment ring that slides radially in yokes. The yokes are arranged so as to allow the bottom abutment ring the possibility of limited movement in the plane of the abutment ring.
That bottom abutment device is advantageous, but it can be difficult to arrange on certain rotors.
Specifically, the pitch of the blades may be controlled by a pilot by means of pitch rods. The pitch rods extend substantially parallel to the axis of rotation of the rotor, extending upwards in elevation from a set of controlling swashplates to the blades.
The pitch rod of a blade may be arranged close to the axis of rotation of the rotor so as to be situated on the flapping axis of the blade, for example. Such an arrangement tends to avoid coupling between the flapping movements and the pitch movements of the blade.
Unfortunately, a bottom abutment ring may present a diameter that is large in order to be capable of facing the corresponding abutment track. Such a large-diameter droop restrainer ring runs the risk of coming into contact with a pitch rod.
In order to remedy that problem, the diameter of the bottom abutment ring may be minimized. Under such circumstances, the corresponding abutment track is mounted on a connection member that is elongate in order to cause the bottom abutment ring and the abutment track to face each other. When the blade is performing rotary movements, in particular about its pitch axis, the connection member then runs the risk of coming into contact with the pitch rod.
In order to avoid such interference, the bottom abutment ring can be replaced by a star-shaped plate.
Document EP 0 360 688 describes a bottom abutment member having such a star-shaped plate. The plate then has a central ring surrounding a circular opening.
The plate also has one radial branch per rotor blade. Each radial branch extends from an outer periphery of the central ring parallel to a radius of the central ring. Each branch presents a plane shape that is substantially rectangular and that is of thickness that is substantially constant. A metal contact shoe is removably mounted by a bolted connection to the outer radial end of each branch. The shoe constitutes a wear part that is to come into contact with a low abutment track of a blade.
The plate can be moved radially within a body of revolution. The body of revolution is fastened about the axis of rotation of the rotor inside the rotor mast. The body of revolution defines an annular U-shaped housing that is radially open towards the rotor mast.
The central ring is then held within the housing, with each branch of the plate passing through an opening in the mast in order to project towards an abutment track of a blade.
The U-shaped housing is defined by a bottom annular support plate that is extended in elevation by a tubular chimney opening out at a collar. A top annular support plate is then bolted to the collar. The central ring thus rests on the bottom annular support plate, with an elastically deformable annulus being adhesively bonded to the central ring and to the top annular support plate.
The central ring can then move radially in its plane about an equilibrium position. The amplitude of such radial movements is nevertheless limited by the chimney. In addition, the elastically deformable annulus tends to return the plate into its equilibrium position when the central ring moves away from the equilibrium position.
The use of a star-shaped plate is found to be advantageous for avoiding interference with the pitch rods. The plate may have a central ring of small diameter. In addition, the branches make it possible to place the shoes for interrupting flapping of a blade at a distance from the axis of rotation. Under such circumstances, the blade does not require the use of an elongate connection member of large size that is large and would therefore run the risk of interfering with the pitch rod.
However, document EP 0 360 688 proposes arranging the plate within a rotor mast. It can be difficult to remove such a plate.
Documents U.S. Pat. Nos. 5,316,442, 4,289,448, and 4,749,339 are also known.